A common problem in paper and pulp processing systems is biofilm, or slime, formation on surfaces of the system components. Biofilm is caused by bacteria in the various process waters in the system. Bacteria in the water can exist in either a free-floating form (known as planktonic) or can be attached to surfaces (known as sessile). Certain bacteria in the process waters such as Deinococcus and Meiothermus prefer the sessile state and are particularly effective biofilm formers. These bacteria, if present in sufficient amounts, can quickly attach to system surfaces and build up to undesirable levels.
Biofilm causes several problems in these systems. For example, biofilm masses that detach from system surfaces can be carried into the pulp waters and formed into the paper sheet. The biofilm masses weaken the formed paper sheet and can cause it to tear or cause holes in the paper. Clearing the tears or removing the damaged sections results in system down-time, lost paper product, reduced efficiency and increased costs. It is therefore desirable to both minimize bacteria in the process waters and to prevent biofilm formation on the system surfaces. A traditional method for controlling biofilm problems is to add microbe control chemicals to the process waters.
Halogenated hydantoins, such as bromochlorodimethylhydantoin, are known microbe control agents. Sweeny et al. (U.S. Pat. No. 6,429,181) teaches that partially halogenated hydantoins such as monochlorodimethylhydantoin (MCDMH) are effective at killing microbes in pulp and paper systems without adverse effects on the chemical additives used in the system. Halogenated hydantoins are effective at killing bacteria in the sessile state and preventing slime formations, but are more expensive than some other known methods of chemical microbe control.
Haloamines, such as chloramines and bromamines, are also known chemicals for microbe control. Haloamines can be formed by combining an ammonium source, such as ammonium sulfate, ammonium chloride, ammonium bromide, ammonium phosphate, ammonium nitrate or any other ammonium salt, including urea, with an oxidant such as sodium hypochlorite. Haloamines are less expensive to produce than halogenated hydantoins and are therefore becoming a more preferred chemical for microbe control of paper and pulp processing systems. Haloamines are effective at minimizing planktonic bacteria levels in the process waters and preventing slime formation on system surfaces, but when in their vapor phase can be very corrosive to system components. The evaporation tendency of haloamines can be orders of magnitude greater than that of sodium hypochlorite.
Other types of chemistry controls, such as chlorine dioxide, can also be used for microbe control. Chlorine dioxide is a good biocide since bactericidal efficacy of ClO2 is not substantially influenced by pH, and ClO2 does not leave toxic disinfection by-products. Chlorine dioxide, however, when dosed in process water remains in gaseous form and thus suffers from the same gas phase corrosion issues as haloamines.
In addition, it has been found that the bacteria that remain in haloamine or chlorine dioxide treated systems, such as in low-circulation chests, are some of the worst slime-formers. In the cases where haloamine or chlorine dioxide chemistry has lost microbe control, a rapid major slime outbreak has occurred. Typical reasons for loss of control include feed equipment failure or under-dosing to reduce cost.
Corrosion is a particular concern in the “short loop,” or short circulation section, of a paper machine, and in the subsequent press and drying section. In a typical pulp and paper process, pulp stock is passed into a headbox, which distributes the pulp stock onto a moving wire in a forming section. The paper sheet is formed in the forming section and then sent to presses and dryers for finishing. The short loop is a system that re-circulates and recycles excess water from the pulp stock. The excess water is collected in a wire pit in the forming section and then a major portion thereof is recirculated back to the headbox for re-use. Although many tanks, lines and other immersed structures of pulp and paper systems are typically formed from acid-proof stainless steel, many components above the water surface level, and in the press and dryer section, are formed from milder steel materials. Especially these components are thus adversely affected by gas phase corrosion when haloamine or chlorine dioxide chemistries are utilized for microbe control.
In practice, the cost savings that result from using haloamines or chlorine dioxide for microbe control overcomes the gas-phase corrosion concerns in these systems. Nevertheless, it would be desirable to employ a chemical method for microbe control that benefits from the cost savings achievable through the use of haloamines or chlorine dioxide and that simultaneously minimizes gas-phase corrosion of steel components of the machine.